Heretofore, rubber-reinforced thermoplastic resin compositions such as an ABS resin have been widely used as general-purpose resins having well-balanced impact resistance, moldability, surface gloss and other mechanical properties and materials usable as a substitute of engineering plastics.
For such a rubber-reinforced thermoplastic resin composition, impact resistance, moldability and other mechanical properties under ordinary conditions as well as these physical properties during high-temperature processing or high-temperature retention, i.e., thermal stability, are very important. When rubber-reinforced thermoplastic resin compositions are extrusion-granulated or molded, deterioration by heat generated by shear or heat retention can often occur, such as decrease in impact resistance of a molded product, yellowing of resin, and deterioration of surface gloss. Further, when heat retention proceeds, a gel-like material is produced in the extruder or molding machine and incorporated in the molded product as a contaminant in the resin, causing problems. Moreover, when a large-sized product is molded or a product having a complicated shape or small thickness is molded, the resin is often molded at a high temperature so that it can be easily extended all over the interior of the mold. In this case, problems often arise such as decrease in impact resistance, fluidity and surface gloss.
Further, the foregoing decrease in fluidity during high-temperature processing, decrease in impact resistance during high-temperature retention, coloration during high-temperature retention, and gelation during high-temperature retention remarkably appear when particles of a rubber polymer having a large particle diameter designed to have an excellent impact resistance under ordinary extrusion molding conditions are used or when particles of a rubber polymer having a low gel fraction content are used. Therefore, it has been difficult to obtain a rubber-reinforced thermoplastic resin composition excellent in impact resistance under ordinary extrusion molding conditions, fluidity during high-temperature processing, impact resistance during high-temperature retention, resistance to coloration during high-temperature retention, and resistance to gelation during high-temperature retention, when the particles of a rubber polymer have a large particle diameter or a low gel fraction content.
In order to solve these problems, there has been proposed to reduce the added amount of the rubber polymer or to increase the gel fraction content of the rubber polymer. However, these approaches are undesirable in the design attaining high impact resistance.
Alternatively, there has been proposed to increase the percent graft amount of a vinyl compound graft-polymerized to the particles of a rubber polymer shown by the following equation: ##EQU1## This approach can maintain surface gloss at high temperatures but disadvantageously causes decrease in impact resistance or fluidity at ordinary molding temperature.
Further, there has been proposed to employ an approach which comprises adding various oxidation inhibitors during extrusion or molding to inhibit the coloration or decrease in impact resistance due to heat deterioration. However, this approach cannot always inhibit the decrease in impact resistance or fluidity at high temperatures or the decrease in impact resistance during high-temperature retention. For the design attaining good impact resistance and surface gloss, U.S. Pat. No. 4,009,227 employs a combination of particles of rubber having a middle to large particle diameter with particles of a rubber polymer having a small particle diameter. However, this approach requires a complicated preparation process which comprises separate graft polymerization steps of particles of rubber having a large particle diameter and the particles of rubber having a small particle diameter, followed by mixing.
JP-A-62-164707 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a thermoplastic resin composition excellent in impact resistance and surface gloss at high temperatures containing particles of rubber having a vinyl compound grafted thereon to an average thickness of from 10 to 20 nm. However, the graft polymerization of a vinyl compound on particles of a rubber polymer generally features that the greater the particle diameter of the rubber polymer is, or the lower the gel fraction content of the rubber polymer is, the more difficultly can be graft-polymerized on the surface of the rubber polymer the vinyl compound. Further, the present inventors have found that in the case where the particles of a rubber polymer has a low gel fraction content, and in the case where, even if the gel fraction content of particles of a rubber polymer is close to 100%, when the particles have a large particle diameter exceeding 250 nm, the thickness of the vinyl compound graft-polymerized on the surface of the particles of a rubber polymer can be easily uneven, making it difficult to balance the impact resistance with the surface gloss at high temperatures only by controlling the average thickness of the vinyl compound graft-polymerized on the particles of a rubber polymer. JP-A-62-164707 also does not refer to fluidity at high temperatures, coloration during high-temperature retention, impact resistance during high-temperature retention, and resistance to gelation during high-temperature retention.
Therefore, a rubber-reinforced thermoplastic resin composition has been hardly provided that is excellent in impact resistance under ordinary extrusion molding conditions, in fluidity during high-temperature processing, impact resistance during high-temperature retention, resistance to coloration during high-temperature retention, on conditions that the particles of a rubber polymer have a large particle diameter and a low gel fraction content, to have an excellent impact resistance under ordinary extrusion molding conditions.